The present invention relates generally to photoplotting machines in which a photosensitive surface such as a sheet or strip of photographic film is exposed to a moving light beam and more particularly to aperture wheels for use in such machines for selectively varying the characteristics of the moving light beam.
Aperture wheels for use in photoplotting machines for selectively varying the characteristics, such as the cross-sectional size and/or shape of a moving light beam are well known in the art.
An example of such a wheel and a machines in which such a wheel is used may be found in U.S. Pat. No. 3,330,182, which patent is incorporated herein by reference. Other examples of such wheels and machines may be found in U.S. Pat. No. 3,548,713 and U.S. Pat. No. 3,610,119.
In the past, these wheels have comprised a generally circular frame which is adapted to be supported on a vertical shaft for rotation about its central axis and having thereon a plurality, such as twenty-four, of circumaxially shaped openings. An aperture plate is removably secured to the frame over each one of the openings by means of two locating pins which are precisely located relative to each opening so that the aperture plate will be accurately positioned over the opening and two screws which engage a pair of holes formed adjacent to each opening. To obtain precise angular indexing of the wheel so that the individual aperture plates may be selectively positioned accurately in axial alighment with the light beam a plurality of locating pins are mounted on the frame, each pin being precisely located relative to one of the circumaxially spaced openings. In using these wheels, the aperture plates are first loaded onto the frame. After loading, the frame is mounted on the machines. After a particular job has been completed, the wheel is removed from the frame and the aperture plates removed from the frame. A new set of aperture plates are then mounted on the frame or the same set of apertures but arranged in a different order are mounted on the frame for the next job. As can be appreciated, during the time interval that the aperture plates are being changed, the machine cannot be used. In order to avoid this time delay, it has become a practice in some instances to provide a plurality of wheels for use with one machine, each wheel being loaded with a different or differently arranged set of aperture plates to meet the requirements of a particular job. The problem with this approach is that because of the extensive machinery required in fabricating the wheels, the cost of the individual wheels is rather expensive.
Accordingly, the need exists for a new and improved aperture wheel for use in a photoplotting machines, especially in applications where the aperture plates must be repeatedly changed.